1. Field of the Invention
This invention relates generally to a process for mounting chip type circuit elements on printed circuit boards and an apparatus therefor. More particularly, the invention relates to such a process and an apparatus which are adapted to organize a production line of high efficiency.
2. Description of the Prior Art
Conventionally, the mounting of chip type circuit elements on printed circuit boards in an organized production line is generally carried out in a manner such that a mounting head which is adapted to hold at least one chip type circuit element thereon by suction is fixed at a position at which a chip type circuit element is to be mounted on a printed circuit board (hereinafter referred to as "circuit element mounting position" or "mounting position") or in a manner such that a mounting head is provided so as to be movable in X-Y directions by NC control or the like to extract chip type circuit elements one by one and be transferred to the circuit element mounting position for the mounting.
However, the former procedure has disadvantages that a production line organized is very long and it is extremely difficult to replace a mounting head. The latter procedure is typically taught in U.S. Pat. No. 4,372,802, issued to Harigane et al, on Feb. 8, 1983. As seen from the aforementioned U.S. patent, it also has a disadvantage that it fails to shorten the time required to mount one chip type circuit element on a printed circuit board. Also, it has another disadvantage of rendering the manufacturing cost of a circuit element mounting apparatus high.
There is another conventional mounting procedure which has been practiced for the organization of a production line. It is adapted to use a circuit element mounting apparatus which is constructed in a manner such that a plurality of magazines having chip type circuit elements different in type from one another received therein, respectively, are arranged so as to positionally correspond to the respective circuit element mounting positions of a printed circuit board, so that a plurality of the circuit element different from one another are simultaneously mounted on the printed circuit board by means of a plurality of vacuum suction pads. The conventional apparatus of such construction effectively shortens the time required to carry out the mounting. However, it has the disadvantage of rendering the lot-to-lot replacement of chip type circuit elements highly troublesome. Also, it has another disadvantage that the use of the magazines causes the apparatus to be considerably complicated in structure.
A further conventional circuit element mounting procedure has been proposed which comprises the steps of progressively feeding chip type circuit elements from a plurality of circuit element feeders according to a predetermined mounting order and mounting the circuit elements one by one on a printed circuit board by means of a mounting head while moving the printed circuit board by means of an X-Y table. Unfortunately, such a conventional procedure is no suitable for use for the organization of a production line, because mounting apparatus used therefor is very large in size and must include the X-Y table.
A chip type circuit element mounting head used in a conventional mounting apparatus is constructed in a manner such that the vertical movement of a suction pin is carried out by means of air cylinders and the sucking-up of a chip type circuit element by the suction pin is carried out utilizing a vacuum suction tube which communicates with the suction pin separately from the air cylinder, and to which an air filter and a vacuum valve are connected. The air filter acts to prevent foreign matter from entering the vacuum valve, thereby preventing damage of the valve by such matter. The vacuum valve acts to close and open a vacuum suction passage communicating with the suction pin to permit the suction pin to carry out the sucking-up and releasing of a chip type circuit element. However, the mounting head of such construction has the disadvantage of causing a circuit element mounting apparatus in which the mounting head is to be incorporated to be highly complicated in structure, to thereby be expensive and large-sized.
A mounting head used in conventional mounting apparatus is divided into two types. One is the single suction pin type, or the type having a single suction pin vertically moved by an air cylinder, and the other is the multiple suction pin type, or the type having a plurality of suction pins vertically moved by air cylinders provided corresponding to the suction pins. In each type of mounting head, much time is required to supply air to the air cylinder to permit a piston on which the suction pin is carried to be vertically moved against static friction in the cylinder. Also, the piston once move is actuated with a relatively high speed because dynamic friction is smaller than static friction, and the high speed actuation is continued right before the stop of the piston. Such characteristics tend to obstruct the sucking-up of a chip type circuit element by means of the suction pin or the mounting of a sucked-up circuit element on a substrate or printed circuit board. More particularly, in order that the suction pin be vertically moved to suck up a chip type circuit element and mount the sucked-up circuit element on a substrate or printed circuit board without applying large acceleration to the circuit element, it is desired that the movement be carried out to start at a relatively low speed, reach maximum speed at the intermediate and greatly decrease in speed just prior to termination. However, the air cylinder used in the conventional mounting head is not constructed to accomplish such movement of the suction pin. Accordingly, in the conventional mounting head, the operating speed of the air cylinder is set sufficiently low to permit the sucking-up and holding of a chip type circuit element by means of the suction pin to be positively carried out, resulting in the operating efficiency being substantially deteriorated.
Furthermore, a conventional detection mechanism for detecting the sucking-up of a chip type circuit element by means of a suction pin in each mounting head of single suction pin type is adapted to detect the suction vacuum pressure of the suction pin. However, the conventional detection mechanism has the disadvantage of lacking reliability in the operation, because the vacuum suction pressure is unstable due to air leakage between the suction pin and a sucked-up circuit element. Also, the conventional detection mechanism fails to detect the position or posture of a sucked-up circuit element. More particularly, the conventional detection mechanism fails to distinguish a correct posture, as shown in FIG. 1A, in which a chip type circuit element 58 is sucked-up by a suction pin 46, from a wrong posture of said sucked-up chip type circuit element, as shown in FIGS. 1B to 1D.
Also, in the detection mechanism for a mounting head of the single suction pin type, the stationary arrangement of a plurality of photosensors is required to carry out the effective detection of the presence and posture of a chip type circuit element. In the detection mechanism of such construction, the photosensors are coplanarly arranged at specific positions on the side of a frame base, rather than the side of a mounting head, in a manner whereby they may be angularly shifted from one another, so that the difference in level between light beams emitted from the photosensors may be provided to detect the presence and posture of a chip type circuit element. However, such a conventional detection mechanism having a disadvantage of requiring two or more photosensors and a large space for arranging the photosensors.
In a detection mechanism for a mounting head of the multiple suction pin type, a vacuum sensor is provided with every suction pin. The detection mechanism lacks reliability in the operation and fails to detect the wrong posture of a sucked-up chip type circuit element. In view of such disadvantages, a detection mechanism using photosensors has been proposed. However, in the proposed detection mechanism, it is substantially impossible to ensure a space in which a plurality of the photosensors are to be arranged.